Toward a Self-Consistent Local and Global Strain History for Ganymede

Forming Ganymede's grooves at smaller strain: Toward a self-consistent local and global strain history for Ganymede Michael T. Bland1, and William B. McKinnon1 1Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, Saint Louis, MO 63130 50 total pages including 11 figures Submitted to Icarus January 3, 2014 Proposed Running Head: Forming Ganymede's grooves at smaller strain. Corresponding Author: Michael T. Bland Department of Earth and Planetary Sciences and McDonnell Center for Space Science Washington University in St. Louis Saint Louis, MO 63130 [email protected] phone: (314) 935-4810 fax: (314) 935-7361 2 Abstract The ubiquity of tectonic features formed in extension, and the apparent absence of ones formed in contraction, has led to the hypothesis that Ganymede has undergone global expansion in its past. Determining the magnitude of such expansion is challenging however, and extrapolation of locally or regionally inferred strains to global-scales often result in strain estimates that exceed those based on global constraints. Here we use numerical simulations of groove terrain formation to develop a strain history for Ganymede that is generally consistent at local, regional, and global scales. These simulations reproduce groove-like amplitudes, wavelengths, and average slopes at modest regional extensions (10-15%). The modest strains are more consistent with global constraints on Ganymede's expansion. Yet locally, surface strains can be much larger (30-60%), consistent with observations of highly-extended impact craters. Thus our simulations satisfy both the smallest-scale and largest-scale inferences of strain on Ganymede. The growth rate of the topography is consistent with (or exceeds) pre- dictions of analytical models, and results from the use of a non-associated plastic rheology that naturally permits localization of brittle failure (plastic strain) into linear fault-like shear zones. These fault-like zones are organized into periodically-spaced graben-like structures with stepped, inward-dipping faults. As in previous work groove amplitudes and wavelengths depend on both the imposed heat flux and surface temperature, but because our brittle strength is depth-dependent, we find (for the parameters explored) that the growth rate of topography is initially faster for lower heat flows. Keywords: Ganymede; Tectonics; Ices. 3 1 Background 1.1 Ganymede's surface strain Ganymede's iconic \grooved terrain" consists of tens- to hundreds-of-kilometers-wide swaths of parallel, periodically spaced ridges and troughs that form a complex tectonic patch- work across the surface. Dominated at the smallest scales by apparent normal faults, and at larger scales by periodic ridges and troughs (or horst and graben, see below), the grooved terrain almost certainly formed via lithospheric extension. In contrast, features formed in contraction have not been identified. Thus, Ganymede's surface is apparently dominated by extensional strain (Pappalardo et al., 2004; Collins et al., 2010). This observation has lead to the suggestion that Ganymede has experienced a period of global expansion during either differentiation (Squyres, 1980; Mueller and McKinnon, 1988) or resonance passage (Showman et al., 1997; Bland et al., 2009). Differentiation yields the greatest areal expansion (up to ∼6% (Mueller and McKinnon, 1988)) as high-density ice deep in the satellite's core is brought up to lower pressure and converts to lower-density phases. The inferred age for the grooved terrain of 2 Ga (albeit with large uncertainty) (Zahnle et al., 2003) poses a challenge for the differentiation hypothesis unless it can be delayed until relatively late in Ganymede's history (e.g., Mueller and McKinnon, 1988). The timing problem is over- come by the resonance-passage hypothesis, which can yield up to ∼2% areal expansion of Ganymede as high-pressure phases of ice melt and transition to lower-density liquid water (Showman et al., 1997; Bland et al., 2009). However, such melting is only transient (current tidal heating in Ganymede is negligible), and it's unclear how global compression during the slow refreezing of Ganymede's ice mantle would affect its surface deformation. Furthermore, Ganymede's orbital evolution into the Laplace-resonance is far from certain (see, e.g., Peale and Lee, 2002; Malhotra, 1991; Greenberg, 1987; Yoder, 1979), and need not include passage through the paleo-resonances necessary for tidal dissipation to occur. There are several independent constraints on Ganymede's global, regional, and local strain (see Fig. 1). McKinnon (1981) argued that the increase in Ganymede's surface area 4 must be less than ∼2% (i.e., less than 1% increase in radius) based on the observation that Galileo Regio, a large, roughly-circular region of dark terrain, retains its intact shape. Radial expansions larger than 1% should have resulted in obvious, circumferential, extensional deformation (e.g., graben) within the Regio (Fig 1A). This observational constraint is consistent with the degree of global expansion suggested by the theoretical considerations described above. In contrast, structural analysis of Ganymede's grooved terrain itself indicate larger regional and global strains (Collins, 2006, 2008). Collins et al. (1998) estimated strains of roughly 50% in a portion of the Uruk Sulcus region from structural reconstruction of grooved terrain swaths. More general estimates of the strain required to produce large-scale grooves suggests 25-100% extension may be typical (Collins, 2006); however, images with resolution sufficient to reconstruct pre-deformation surfaces are limited. Lower-amplitude grooves are presumed to require less strain to form (Collins, 2006). Extrapolation of these strain magnitudes to the rest of the satellite suggests that Ganymede underwent an areal increase of 6 - 20%, with a nominal value of 8% (Collins, 2008, 2009) (Fig. 1C and D). The low-end estimate may be consistent with strain magnitudes resulting from differentiation; however, these estimates are generally larger than those based on Galileo Regio, and exceed the global strains resulting from internal melting. To date these difference have not been fully reconciled. At the smallest scales, surface strain on Ganymede can be quite large. Deformed craters on Ganymede, whose initially circular shape lends itself to strain analysis without requiring an assumption of tectonic structure, indicate that localized strains can exceed 100% (Pap- palardo and Collins, 2005) (Fig. 1B). Many of these measurements were performed in craters 20-30 km in diameter with extensional zones ∼10 km wide. Despite this evidence for large local strains, the majority of Ganymede's grooves must have formed at lower extensional strain (Collins, 2008, 2009). Bubastis Sulcus near Ganymede's south pole is ∼600 km across and includes large-amplitude grooves (Fig. 2). Forming the Bubastis grooves at 100% extension of the original surface would require a 300 km increase (1.8%) in Ganymede's circumference, or a 3.6% increase in Ganymede's surface area for Bubastis Sulcus alone. The strain mag- 5 nitude exceeds that which can be produced by melting and is nearly half that produced by global expansion. Whereas Bubastis is one of Ganymede's widest groove swaths, invoking strains of 50-100% wherever large-amplitude grooves are observed would result in cumulative global strains that well exceed current estimates. This observation alone suggests that even large-amplitude grooves must be able to form at relatively small regional strains (∼10%), or that large amounts of hitherto unrecognized crustal consumption has occurred. 1.2 Salient features of the grooved terrain Truly constraining Ganymede's strain history requires a detailed understanding of the formation of its dominant extensional tectonic feature: the grooved terrain. Nearly two- thirds of Ganymede is composed of bright terrain, much of which includes tectonic structures dubbed grooves (Patterson et al., 2010). The remaining third of the surface is heavily- cratered, lower-albedo terrain. In generic terms, the grooved terrain consists of swaths of periodically spaced ridges and troughs with amplitudes of several hundred meters (Squyres, 1981; Giese et al., 1998; and Fig 2). The salient feature of the grooved terrain is the strong periodicity of the ridges and troughs within a single groove swath. Across the satellite these wavelengths vary from 3-17 km from one groove swath to the next (Grimm and Squyres, 1985; Patel et al., 1999). At large spatial scales topographic slopes are generally low, giving the grooved terrain an undulatory character (Squyres, 1981; Giese et al., 1998), though regional variations exist (see Fig 2b). In addition to the large-scale grooves first observed in Voyager images, higher-resolution images from Galileo have revealed complex, finer-scale lineations (presumably fractures) with periodic spacings of order 1 km (Pappalardo et al., 1998; Patel et al., 1999). A detailed review of grooved terrain (and Ganymede's tectonics in general) is provided in Pappalardo et al. (2004) and Collins et al. (2010). The general description above belies the complexity and variability of bright terrain morphologies present on Ganymede. Patterson et al. (2010) subdivided the bright terrain into three basic morphologies: grooved, subdued, and irregular. Grooved terrains, the focus of this paper, consist of lanes of large-amplitude, evenly-spaced ridges and grooves. Both groove

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